An εHf and δ18O Isotopic Study of Zircon of the Mount Osceola and Conway Granites White Mountain Batholith New Hampshire: Deciphering the Petrogenesis of A-Type Granites

摘要

A-type granites form in anorogenic settings and typically have high REE concentrations, K2O, Na2O, SiO2, FeOtotal, but low contents of Al2O3, MgO, CaO compared to other granite types. They have been divided in two groups according to their geochemical characteristics: differentiates of mantle-derived magmas (A1), and granites that are the result of melting depleted, lower crust (A2).The two largest A-type granites of the Mesozoic White Mountain Batholith of New Hampshire are the Mount Osceola and Conway granites. Electron microprobe analyses of biotite and amphibole in both granites are similar to those in other A-type granites: Fe-rich, but low MgO, and Al2O3. Whole-rock major and trace elements compositions of the Mount Osceola and the Conway granites are similar; both have high contents of REE, Zr, Nb, high Nb/Y ratios, and low CaO, Eu, and Sr and other compatible elements. Based on their high Nb/Y ratios, both granites are classified as mantle-derived magmas (A1). Microanalyses of?18O and ?Hf of zircon show significant crustal contamination in both granites. The ?18O values for zircons from the Mount Osceola are between 7.4-8.9‰, and for the Conway Granite are 7.0-8.1‰. These values are distinct from mantle zircon (?18O 5.3±0.3‰), which indicates large degrees of crustal contamination in both granites. Additionally, ?Hf (188Ma) for the Mount Osceola zircon ranges from -1.1 to +3.4, and those from the Conway Granite range from -2.1 to +4.6, indicating magma derivation in depleted mantle (?Hf 0) along with a crustal component. Although both granites have A1 compositions suggesting a mantle-derivation, this simple process is not recorded by the zircons. These zircons crystallized after considerable crustal contamination of mantle-derived A1 magmas and missed capturing the signature of that mantle component.